Below are two explanations of why the concept of once saved, always saved is not scriptural. They are taken from the WELS Q & A site and are written by theologian professors of a confessional Lutheran seminary:
Scripture makes it absolutely clear we cannot contribute the least speck to our salvation. Our baptism clothed us in Christ’s perfect righteousness (Galatians 3:27). By his Son’s life, death, and resurrection God declared us—by nature wicked—to be righteous in his sight (Romans 4:5). Even our faith, which clings to Christ’s salvation, is part of God’s complete gift package (Ephesians 2:8,9) worked by the Spirit (1 Corinthians 12:3) through the gospel (Romans 10:17).
You are also correct that salvation can be lost. In the final analysis, it’s only unbelief that damns and not "bad behavior" itself. Yet carelessly persisting in sin destroys faith. In Luke 8 Jesus speaks of those who fall in times of testing and others whose faith is choked by life’s worries, riches, and pleasures. 1 Timothy 1 mentions those who shipwrecked their faith by not "holding on to faith and a good conscience."
We cannot contribute one speck to our salvation, but by our own arrogance or carelessness we can throw it away. Therefore, Scripture urges us repeatedly to fight the good fight of faith (Ephesians 6 and 2 Timothy 4 for example). We can participate in this good fight because the Spirit planted a new self within us when he brought us to faith.
However, this cooperation in sanctification is in no way a meritorious work that partially earns salvation. Our sanctified life does not make us any more children of God than we already are. We are already heirs of heaven in Jesus.
Second, it’s not our good works that preserve faith. Good works aren’t a means of grace. Good works flow from faith worked in us by the means of grace. The most crucial battle of the good fight is living in daily repentance. That’s hardly a meritorious work!
Daily repentance means that the Spirit through his law crushes our natural proud arrogance. Daily we learn to hate what our sinful nature loves. Then daily through the gospel the Spirit cheers our spirit through Christ’s forgiveness that is new every morning. Through daily repentance the same gospel that created our faith preserves and strengthens our faith.
My sins threaten and weaken my faith, but the Spirit through the gospel in Word and sacraments strengthens and preserves my faith. That’s why Lutherans typically speak of God’s preservation of faith and not the perseverance of the saints. The key is not our perseverance but the Spirit’s preservation.
The rest of our sanctified life then flows from this strengthening and preserving work of the Spirit. Fruits of faith don’t strengthen or preserve faith but flow from a faith that has been strengthened and preserved. What is more, although our new self participates in this sanctified living, the praise belongs to the Spirit. That same gospel that preserves our faith graciously empowers our sanctified living. "It is God who works in you to will and to act according to his good pleasure" (Philippians 2:13).
Evil works can lose salvation by destroying faith. But the reverse isn’t true. It isn’t my good works that preserve faith. Keeping my faith has everything to do with the gospel, and that is purely the work of the Holy Spirit.
* * * * *We teach that believers can fall from faith because the Bible teaches that believers can fall from faith. In explaining the parable of the sower our Savior says, “Those on the rock are the ones who receive the word with joy when they hear it, but they have no root. They believe for a while, but in the time of testing they fall away” (Luke 8:13).
Paul writes to the Galatians, “you who are trying to be justified by the law have been alienated from Christ; you have fallen from grace” (Galatians 5:4).
Again Paul writes, “Timothy, my son, I give you this instruction in keeping with the prophecies once made about you, so that by following them you may fight the good fight, holding on to the faith and a good conscience. Some have rejected these and so have shipwrecked their faith” (1 Timothy 1:19). To shipwreck one’s faith is to destroy one’s faith.
The Bible warns us, “If you think you are standing firm, be careful that you don’t fall!” (1 Corinthians 10:12)
To answer your question, one needs to understand the difference between the law and the gospel and the purpose of each. The passages that tell us that believers can fall from faith are law. The purpose of the law is not to comfort us but to warn us. The law will produce anxiety because it reveals our sin and our inability to do what God demands. God issues these warning so that we do not trust in our own strength, merit, or abilities. The purpose of the law is to make us despair of our own ability to save ourselves or contribute to our salvation in any way (Romans 3:19-20). If we rely on our strength or goodness we will fall.
When we are troubled by our sin, our weaknesses, and our inability to contribute to our salvation, we are to look to those wonderful promises of God.
“For it is by grace you have been saved, through faith—and this not from yourselves, it is the gift of God—not by works so that no one can boast” (Ephesians 2:8-9).
“The blood of Jesus, his Son, purifies us from every sin” (1 John 1:7).
“He is the atoning sacrifice for our sins for our sins, and not only for ours but also for the sins of the whole world” (1 John 2:2).
“My sheep listen to my voice; I know them, and they follow me. I give them eternal life, and they shall never perish; no one can snatch them out of my hand” (John 10:27-28).
Those promises give us comfort and strength. Thank God that our salvations lies in God’s hands from first to last (Romans 8:28-39). The Holy Spirit uses that message of God’s love and mercy in Christ to create and strengthen faith (Romans 10:13-17). It is in the promises of God that we find the assurance of our salvation.
We cannot find that comfort and our assurance in human reason or in the law. We find our assurance that we are saved when we hear God’s promises and the Holy Spirit leads us to believe them.
That's why every Christian will want to be faithful in the use of the means of grace (the gospel in God's Word and the sacraments) because it is through these means that God strengthens and preserves faith so that our assurance of heaven is firm. We can find certainty of salvation nowhere else.